Temperature is an important basic physical quantity to measure, in engineering applications and research. The techniques for measuring temperature may be classified into contacting and non-contacting measuring.
The non-contacting method mainly deduces temperature according to radiant heat or a thermal image. Advantages of the non-contacting method include the capability of measuring a very high temperature without affecting the temperature of the subject concerned. However, the emissivity of the measured subject must be known beforehand, so a body with an unknown emissivity cannot be measured with the technique. Furthermore, the method is limited to measuring the surface temperature and the accuracy is not good (about ±10° C.). Moreover, the non-contacting technique is not suitable for measuring medium to low temperatures.
The contact measuring technique mainly employs a temperature probe to closely contact with a measured matter and deduces temperature according to a pre-calibrated relationship relating the physical characteristic of a probe to temperature. Common contact measuring sensors are gas thermometer, alcohol thermometer, mercury thermometer, thermocouple, thermistor, and resistance thermometer. An advantage of the contacting measuring is that the measuring position is at will. The drawbacks of gas/alcohol/mercury/thermometer are: 1. slow respond rate; 2. narrow measuring range; 3. low accuracy (about 0.5° C.); 4. automatic data acquisition is difficult. Moreover, a temperature of the measured matter may be changed due to the large heat capacity of the probe. Therefore, electrical thermometer, comprising thermocouple, thermistor, and resistance thermometer, is usually used in the industry. The accuracy of the thermistor, and resistance thermometer is high but the measuring range is narrow. Thermocouple is cheaper and has a wider measuring range, but its accuracy is lower (about 0.5° C.). In addition, the thermocouple uses dissimilar metal junctions to get the thermo-electromotive force for deducing the temperature. Problems of environment interaction and galvanic corrosion may arise and affect the stability of measurement. It is also prone to electromagnetic interferences, and is not suitable for use in the presence of ionizing radiation. A problem common to all conventional measuring technique is the impracticality of embedding the probes in a structure to measure interior temperature without affecting structural integrity.
With regard to measurements of the torsional force and axial force, the conventional arts may employ resistance strain gauge, piezoelectric sensor capacitance or an inductance. There are easily affected by electromagnetic interference and are difficult to be embedded into a structure to measure the interior strain without degrading the structure integrity. Other methods, such as: photoelasticity and optic interference/diffraction need to employ more complicated, expensive, and massive equipment and are difficult to automate. As a result, they are not convenient for practical engineering applications. Periodic variation of refractive index in the core of an optical fiber can be formed by side writing using a UV laser beam with a proper amplitude grating mask. The resulting long period fiber grating can attenuate specific wavelengths and is used as filter in optical communication applications. Long period fiber grating can also be manufactured by etching a fiber to give a corrugated surface with the correct period. The specific wavelengths that will be attenuated will vary with temperature and deformation of the fiber. By measuring the change in the characteristic attenuation wavelengths, physical quantities such as temperature variations, axial, transverse, and torsional deformation can be deduced.
On the other hand, a short period fiber Bragg grating will reflect a characteristic narrow band wavelength. This characteristic wavelength will also vary with temperature and fiber deformation and so can also be employed to measure these physical quantities.
At present, interrogating the wavelength shifts of the long period fiber grating and the short period fiber Bragg grating signals requires the use of complicated instrument, such as optical spectrum analyzer or Fabry-Pèrot interferometer. These instruments are expensive and the measuring rate is slow. It is difficult to employ these instruments to measure a fast changing physical quantity.